Conventional storage systems include an inductive head that uses an inductive element to write information onto a recording surface of the magnetic medium, such as a magnetic disk. The inductive element usually includes an inductive coil that writes information by creating a changing magnetic field near the magnetic medium. A write driver circuit is connected to the magnetic head at two head terminals. During writing operations, the write driver circuit forces a relatively large write current through the inductive coil to create a magnetic field that polarizes adjacent bit positions on the recording surface. Digital information is stored by reversing the polarization of selected bit positions which is done by reversing the direction of the current flow in the inductive coil.
The typical write driver circuit includes an "H-bridge" for controlling the direction of current flow through the inductive coil. The H-bridge includes upper "pull-up" bipolar transistors and lower "pull-down" bipolar transistors. The upper bipolar transistors are connected between a first supply voltage and the head contacts or terminals. The lower bipolar transistors are connected between the head terminals and a second supply voltage through a write current sink. The write driver circuit controls the direction of current flow through the inductive coil by driving selected transistors in the H-bridge between ON and OFF states.
The rate at which information can be stored on a recording surface through an inductive head is directly proportional to the rate at which the direction of current can be reversed in the inductive coil. The rise/fall time of the inductive coil is approximately: EQU di/dt=V/L
where di/dt is the rate of change of the current over time across the inductive coil, V is the available voltage across the inductive coil, and L is the inductive load. Therefore, the speed of the H-bridge is directly proportional to the available voltage across the inductive coil. The available voltage is determined by subtracting the voltage drops across the pull-up transistors, the pull-down transistors, and the write current sink from the supply voltage.
The write circuit is a portion of a preamplifier system. The preamplifier system also includes a read circuit which, together with the write circuit, reads and writes information to and from the magnetic medium.
A preamplifier system is connected to the magnetic head coil at the head contacts.
The lower switches of the H-bridge are generally controlled by FET transistors. These transistors are formed by a CMOS process.
FIG. 2 illustrates the overshoot and ringing effects on the head current versus bit times between transitions. FIG. 2 illustrates the effect of overshoot by the fact that the head current rises to over 80 milliamps when a steady state value of 50 milliamps is desired. Thus, the overshoot is over 30 milliamps. Further, FIG. 2 illustrates the effects of ringing through four different bit times. Each bit time represents a period of time when another transition on the disk could be written. The ringing is the dampened sinusoidal effect after the first bit time. FIG. 1 illustrates a graph of head current versus time and the associated jitter resulting from a transition being written at a bit time. Depending on the initial head current, a different zero crossing time is achieved. Thus, the jitter is caused by write transitions beginning at current values that depart from the desired head current. Assuming the rise and fall times to be constant, transitions beginning at different values of the head current will cross the slicing threshold at different points in time, resulting in the jitter illustrated in FIG. 1. Thus, when a waveform has significant overshoot and ringing, the head current is varying significantly with time. If the overshoot and ringing are kept to a minimum, the waveform values will vary less over time, resulting in reduced jitter.
Overshoot can be explained more fully with respect to the idealized model of a head current illustrated in FIG. 9. In FIG. 9, a resistor 902 represents the resistance associated with the coil in the head. The inductor 904 corresponds to the inductance of the coil. The capacitor 906 is associated with the capacitance found in a disk drive system that affects the head circuit. Typically, the preamplifier is mounted on a flex which includes long lines to the head. The current generator 908 generates current that is introduced into the H-bridge across the head. As the current is first sourced into the head, the current flows into capacitor 906 to build a voltage across the head; the current in L cannot change instantaneously. The inductor current then begins to build due to the voltage across it according to the equation ##EQU1##
The voltage across the head continues to build until the inductor current reaches the desired value. The voltage that remains across the head causes the inductor current to continue to build to a value in excess of the desired current, resulting in overshoot. As discussed previously, the overshoot is significantly large and is undesirable.